The present invention relates to a process for attaching a thermally and electrically conductive lead to a refractory metal contact of a semiconductor device by a high temperature brazing process.
Passivated semiconductor devices generally include a semiconductor body composed substantially of silicon, a layer of passivating material such as glass or plastic disposed about the semiconductor body, and at least one metallic contact extending outwardly from the semiconductor body through the passivating layer as an external contact for connection with associated circuitry. More specifically, such devices require that the metallic contacts be refractory in nature in order that the coefficients of thermal expansion of the semiconductor body, the passivating layer and the metallic contact be reasonably matched to avoid breakage during thermal cycling. Molybdenum, tungsten, tantalum and various special alloys are typical of the refractory metals used as such refractory metal contacts; however, since such materials are both expensive and relatively poor conductors of both heat and electric current, the refractory metal contacts are generally joined to good conventional conductors (such as copper, silver or various special alloys) just beyond the passivating layer, the connection between the refractory contact and the other circuitry elements being made by the conventional conductor. In the case of axial-lead construction semiconductor rectifiers, the connection of the conventional axial lead to the refractory metal contact is accomplished by one of the following two procedures.
In the first procedure, the refractory material is initially plated with a solderable metal such as silver before application of the passivating layer. After the passivating operation, the axial leads are attached to the refractory metal contacts using "soft solder" preforms with melting points typically less than 300.degree.C. Devices containing such connections have the disadvantages commonly associated with soft solder contacts. If extreme temperature control is not exercised in soldering the axial lead to other circuit components, the axial lead may detach from the refractory metal contact as the soft solder heats up. In any case, it has been found that such soft solder joints are subject to thermal fatigue and a resultant short operating life.
In the other procedure, the refractory metal contact is joined to the axial lead by a special welding process known as butt welding. The joint and the axial lead must thereafter be exposed to all the chemical and heat-treating processes subsequently required to (1) join the semiconductor body to the refractory metal contact, (2) etch the subassembly, and (3) apply and fuse the passivating glass layer. The devices fashioned according to this procedure are not reliable in the first place because a true weld is not possible between the refractory metal of the contact and the conventional conductive metal of the axial lead. Furthermore, the processing operations required to complete the semiconductor device subsequent to formation of the refractory contact/axial lead joint frequently result in a weak and porous joint which will develop high electrical and thermal resistance in time or eventually even fail mechanical and fall off. Experience has shown that butt welded joints have extremely high failure rates when exposed to conditions of high temperature and high humidity, the failure rate rising as high as 50 percent at 85.degree.C and 85 percent relative humidity.
Accordingly, it is an object of the present invention to provide a process for attaching a thermally and electrically conductive lead to a refractory metal contact of a semiconductor device using a high temperature brazing process.
It is another object to provide such a process which avoids the aforementioned disadvantages of soft solder and "butt welded" joints.
It is also an object to provide such a process wherein the resultant joint is of higher physical strength and lower porosity than a butt-welded joint, withstands a higher temperature than a soft solder joint, and withstands conditions of high temperature and high humidity better than a butt-welded joint.
It is a further object to provide such a process which enables a conventional lead to be joined to a refractory contact at the same time and at the same temperature conditions as the refractory contact is joined to the semiconductor body.
Yet another object is to provide such a process which utilizes a special preform, is simple and economical to perform, is less expensive than a butt welding, and provides joints superior to those produced by soft solder or butt welding processes.
Yet a further object is to provide such a process whereby a joint between a semiconductor body and a refractory contact member is formed at the same time and at the same temperatures as the joint between the refractory contact member and a lead member.
A final object is to provide a semiconductor device having a lead member joined thereto by the aforesaid process.